Electrode contacts of medical devices, such as cochlear implants have capacitive and resistive aspects of the interface between the electrode contact and the tissue, and these values are worth knowing for several reasons, e.g. detection of high impedance or short circuit electrodes, decision not to use electrodes, model calculations in research, etc. Electrode tissue interfaces commonly are based on purely resistive models. But it is known that there are also capacitive contributions to the impedance and that the interface behaves in a nonlinear way. See, for example, Ragheb, T. & Geddes, L. A., “The Polarization Impedance Of Common Electrode Metals Operated At Low Current Density,” Annals of Biomedical Engineering (ABME), 1991, 19, 151-163; Lai, W. & Choi, C. T. M. “Incorporating the Electrode-Tissue Interface to Cochlear Implant Models,” IEEE Transactions on Magnetics, 2007, 43, 1721-1724; Fridman, G. Y. & Karunasiri, R. T., Removing Artifact in Evoked Compound Action Potential Recordings in Neural Stimulators,” US Patent Application 20070244410; all of which are hereby incorporated by reference.
When a malfunction of an implantable medical device is suspected—for example, a stimulation malfunction in a cochlear implant—then information about the actual operation of the system is needed. There are various existing approaches to detecting malfunctions in implanted medical devices. For example, external amplifiers have been used to detect stimulation pulses by attaching external amplifier electrodes to the skin and synchronizing the signal recording with application of the suspicious stimulation signal. Some disadvantages of this method are that the equipment often is not locally available and has to be shipped, a specialist has to do the recordings, and the recording itself is not easy to do because of the skin electrodes and the necessary synchronization.
Another approach is based on the use of “telemetry” recordings where the implant itself records the voltage of the current source during a stimulation pulse. See, for example, U.S. Pat. No. 5,938,691, “Multichannel Implantable Cochlear Stimulator,” which is hereby incorporated by reference. This can work well, but has some limitations (depending on the type of implant) such as only providing information about the resistive component of the current path over the electrodes.
Telemetry data has also been used U.S. Patent Publication 20060247735 by Honert described measuring stimulation currents for stimulating and non-stimulating electrodes to produce a matrix of impedance values. The impedance matrix is used to calculate electrode currents that are required to produce a desired pattern of stimulation voltage in the cochlea.